Implantable medical devices are employed in various applications. For example, an implantable cardiac device may perform one or more functions including sensing signals generated in a patient's heart, pacing the heart to maintain regular contractions, and providing defibrillation shocks to the heart. Similarly, an implantable stimulation device may be used to apply stimulation signals to a patient's muscular tissue, neurological system, or some other area of the patient's body.
In many cases, there is a need to communicate with an implantable medical device after it has been implanted in a patient. For example, an external monitoring device located in an operating room, a doctor's office, a clinic, a person's home, or some other suitable location may be used to retrieve information collected by and/or stored in the implanted medical device. In the case of an implanted cardiac device, such information may include sensed cardiac activity data that a treating physician may analyze to learn about the patient's health. Similarly, an external programming device (which typically includes monitoring capabilities as well) may be used by a treating physician to change the operating parameters of the implanted medical device. Such parameters include, for example, the timing or magnitude of stimulation pulses generated by the implanted medical device.
Typically, radiofrequency (RF) telemetry is employed for communication between an external device and an implanted medical device. In such a case, the external device and the implanted medical device each Include an RF transceiver that transmits and receives RF signals (e.g., via a designated medical band).
In practice, communication problems may be encountered when an external device and implanted medical device attempt to communicate with one another. For example, a transmitting device typically transmits RF signals at low power levels to reduce the possibility that these RF transmissions will interfere with other nearby RF devices (e.g., an external device and/or an implanted medical device located in a nearby room). However, environmental conditions such as objects in the room and nearby noise sources may interfere with the reception of such low power RF signals. Consequently, under certain circumstances, a receiving device may not be able to accurately acquire the information sent via these RF signals. Thus, there is a need for improved techniques for communicating between an implantable medical device and an external device.